The world's oceans comprise a plurality of stratified layers. According to water density, an ocean is divided into three horizontal depth zones: the mixed layer, pycnocline, and deep layer. Where a decline in temperature with depth is responsible for the increase in density with depth, the pycnocline is also a thermocline. On the other hand, if an increase in salinity is responsible for the increase in density with depth, the pycnocline is also a halocline. Typically, the pycnocline extends to a depth of 500 to 1000 m (1600 to 3300 ft). Water from different layers do not mix normally. However, the combination of persistent winds, Earth's rotation (the Coriolis effect), and restrictions on lateral movements of water caused by shorelines induces upward water movements. Coastal upwelling occurs where Ekman transport moves surface waters away from the coast; surface waters are replaced by water that wells up from below. Beside wind-driven upwelling, geostrophic currents also generate upwellings in the oceans.
The term “upwelling” as used herein refers to the upward directional movement of a mass of water from a source to a target within a body of water, such as but not limited to an ocean, an offshore area, a coastal area, or a lake. The source is characterized by its horizontal location (“source position”) and/or its vertical location (“source depth”) in the body of water. The position can be specified by its latitude and/or its longitude, or by its proximity to one or more oceanographic features. The target can be described similarly by its location (“target position”) and/or the depth of the target (“target depth”). Upwelling occurs where the depth of the source is greater than the depth of the target. The mass of water transferred from a source to a target of lesser depth is referred to as “upwelled water.”
In terms of exposure to sunlight, a body of water comprises a photic zone and an aphotic zone. The photic zone is the layer of water in a body of water, such as a lake or ocean, that is penetrated by sufficient sunlight for photosynthesis to occur. It extends from the surface downwards to a depth where light intensity falls to 1% of that at the surface. The thickness of the photic zone depends on the extent of light attenuation in the water column and is thus greatly affected by turbidity. The photic zone can be about a few centimeters in depth in turbid eutrophic lakes and up to about 200 meters in the open ocean. The aphotic zone is the layer of water beneath the photic zone that supports a minimum of photosynthetic activity, if any.
Primary production occurs predominantly in the photic zone where autotrophic organisms, such as microalgae, grow under sunlight and consume the nutrients in the photic zone. Heterotrophic organisms, e.g., zooplankton, feed on the autotrophic organisms, and the food web continues with the predation of phytoplankton and zooplankton by larger organisms, such as fishes, shellfishes, birds, and mammals. The particulate waste products produced by organisms in the photic zone and decaying bodies of dead organisms sink into and enrich deeper water in the aphotic zone. The water in the photic zone generally contains less nutrients than the water in the aphotic zone. Due to the paucity of mixing between surface water and denser water in the deep layer, many nutrients are deposited and accumulated near or at the bottom of a water column. Upwelled water derived from greater depth is thus richer with nutrients than surface water.
In certain embodiments, a nutrient profile comprises the concentration(s) of one or more of the following nutrients: C, N, P, K, and/or minor nutrients, such as Si and Fe, their organic and/or inorganic form, and their dissolved and/or particulate forms. In certain embodiments, the profile can also be expressed in terms of the relative amounts of nutrients, such as a stoichiometric ratio. For example, it can correspond to the Redfield ratio for diatoms which is C:Si:N:P=106:15:16:1. In certain embodiments, an upwelling of water from a greater depth increases the concentrations of C, N, P, K, and/or minor nutrients, such as Si and Fe, at a target near the surface. Many methods well known in the art can be used to determine the concentrations of nutrients in water.
The nutrient profile of the water at a source is different from the nutrient profile of the water at a target before an episode of upwelling. The arrival of upwelled water from the source changes the nutrient profile at the target. The desirability of obtaining upwelled water from a source of greater depth, which is richer with nutrients than water at a target, growing algae in the upwelled nutrient-rich water at the target, and harvesting the algae, is explained in International Patent Publication No. WO 2010/141794 A1, incorporated herein by reference in its entirety. The desirability of pumping devices, which are purportedly useful as sub-sea collectors is described in International Patent Publication No. WO 2004/070165 A1; and the desirability of using Ocean Thermal Energy Conversion (“OTEC”) systems to generate biomass is explained in U.S. Patent Application No. 2002/031823 A1 and U.S. Pat. No. 6,863,027 B2.
Whereas it is desirable to be selective about the nutrients provided to the algal culture at the target, a need exists for an approach to selectively control the sourcing of the nutrients from the deep water, using OTEC, and the like, to selectively control the delivery of these nutrients to the algal culture, and/or to selectively control the feeding of the algal culture to the planktivorious organisms, for example, fish.